Treatment with methimazole in a 3-year-old male with thyroid hormone resistance.

BACKGROUND Thyroid hormone resistance syndromes are disorders in which there is decreased end-organ responsiveness to thyroid hormone. Patients typically present with elevated levels of thyroxine and triiodothyronine with a normal or increased serum thyroid-stimulating hormone concentration. Clinical features are variable, and there are no guidelines regarding treatment. PATIENT The patient was noted to have tachycardia at 6 months of age. He had persistently elevated free thyroxine and normal/mildly elevated thyroid-stimulating hormone for the first 2 years of life. At age 2 years, he was noted to have sinus tachycardia, a mildly enlarged thyroid, hyperactive behavior, subtle developmental delay, and poor weight gain. The patient has a previously described de novo mutation in the thyroid hormone receptor-β gene. INTERVENTION AND OUTCOMES He was started on methimazole (0.3-0.5 mg/kg/day) at age 3 years to treat his symptoms. With medication, weight gain, sleep, behavior, and tachycardia improved. Linear growth has remained appropriate for age. Heart rate is in the upper normal range and his thyroid has become more enlarged. CONCLUSIONS Methimazole has improved thyrotoxic symptoms in a 3-year-old male with thyroid hormone resistance. The use of methimazole should be considered in certain patients.